Method for visual comparison of information



y 5, 1970 B. KAZAN 3,510,660

METHOD FOR VISUAL COMPARISON OF INFORMATION Filed Sept. 29, 1966 4 AC VOLTAGE INVENTOR. BENJAMIN KAZAN FIG. 2

47' OPNEYS United States Patent Ofice 3,510,660 Patented May 5, 1970 3,510,660 METHOD FOR VISUAL COMPARISON OF INFORMATION Benjamin Kazan, Pasadena, Calif., assiguor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Sept. 29, 1966, Ser. No. 582,863 Int. Cl. H01l 17/00 US. Cl. 250-213 16 Claims ABSTRACT OF THE DISCLOSURE An invention relating to electroluminescent devices and, in particular, to an imaging system making use of such devices. Additionally, the present invention relates to a method for the use of such a storage device which involves producing a stored image and one or more nonstored images.

At present, a variety of solid state imaging devices are known but have not received significant utilization because of the practical problems encountered in their operation. The storage action of these devices depends on one of several different phenomenon including the slow decay of conductivity after excitation of a photoconductive material, the hysteresis effect in photoconductors and optical feed back.

One type of solid state imaging device involves a display panel consisting of a layer of variable impedance material in series with a layer of electroluminescent material as described in the patents to Benjamin Kazan US. 2,768,310 issued Oct. 23, 1956 and 2,949,537 issued Aug. 16, 1960. As described therein, the image is produced by the increase in conductivity of the portions of a photoconductive material against which incident radiation impinges. Such conductivity increase produces a correspond ing luminescence in the adjoining portion of the electroluminescent material.

-A further type of solid state imaging device is the hysteresis-type photoconductor panel wherein an electric field is simultaneously applied to the photoconductive material. In this arrangement, the photoconductive material becomes conducting when exposed to a small amount of light, the conductivity remaining at an almost constant level for substantial periods of time instead of gradually decaying after excitation.

In the patent to Benjamin Kazan 2,905,849 issued Sept. 22, 1959, there is shown a storage device having a target comprising a transparent support plate having on one surface thereof a plurality of spaced transparent electrodes, the transparent electrodes being covered in turn by a layer of ferro-electric material which in turn is covered by a plurality of spaced conducting elements. Input signals to the ferro-electric material caused a change in the impedance of that layer resulting in a modification of the voltage drop across the electroluminescent layer. Sufficient increase in the impedance of the ferroelectric layer will cause complete termination of the light emitted from the electroluminescent layer.

In copending application Ser. No. 582,856 filed Sept. 29, 1966, a continuation-in-part application of Ser. No. 514,860, filed Dec. 20, 1965, there is disclosed a new and improved electroluminescent storage device which is not subject to defects which plague the operation of prior known storage panels. The storage device involves a display panel comprising a plurality of spaced electrodes on one surface of a supporting substrate, a layer of electroluminescent material overlying the plurality of electrodes and forming a part of the electrical connection between the electrodes, and a layer of a field effect semiconductor material overlying the layer of electroluminescent material and forming a succeeding part of the electrical connection between the electrodes, the panel having a charge retaining surface adapted to store an electrostatic charge pattern thereon. Such a panel is used in combination with means for depositing a charge pattern on the charge retaining surface. In operation, an alternating current voltage is applied between the spaced electrodes which is suflicient to induce electroluminescence when the field effect semiconductor material is at its low impedance state. It was found that the deposition of an electrostatic charge on the charge retaining surface of the display panel could be used to control the flow of current from electrode to electrode. Deposition of electrostatic charge increases the impedance of the field effect semiconductor thereby reducing or interrupting the flow of current in adjacent areas. Reduction of current flow causes a corresponding reduction in light output from the electroluminescent layer resulting in a half-toned response. If the current is lowered below that which is sufficient to induce electroluminescence, luminescence will not occur and that particular portion of the storage device will appear dark. Conversely, the impedance is lowered and current flow increased as the charges are neutralized or removed from the surface. By selectively placing or modifying a charge pattern on the surface of the display panel an image can be produced and stored for long periods upon the device.

In the operation of solid state intensifier panels employing interdigital electrode structures, as in the storage panels previously mentioned, it has always been the practice to produce and store a single image on the panel for viewing purposes. In many situations, however, it would be desirable, to produce more than one image which can be viewed, for example, for purposes of comparison. Accordingly, it is an object of this invention to provide a method for the production of two or more viewable images on solid state storage panels.

A further object of this invention is to provide a method for the production of a single stored image and one or more non-stored images for viewing purposes on solid state storage panels.

A further object is to provide an improved imaging system for the simultaneously viewing of the more than one image.

The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of specific exemplary embodiments of the invention.

The above and still further objects may be accomplished in accordance with the present invention by providing an electroluminescent device comprising a plurality of spaced electrodes on one surface of a supporting substrate, a layer of electroluminescent material overlying the plurality of electrodes, and a layer of variable impedance material overlying the layer of electroluminescent material; in combination with means for producing and storing an image upon the electroluminescent device, and means for projecting one or more images onto the device while a stored image is present thereon. Suitable variable impedance materials include field effect semiconductors, photoconductors, and ferro-electric material. Broadly, the method for simultaneously producing two or more images on the device comprises producing and storing a first image in a manner disclosed by the aforementioned patents, and thereafter projecting one or more images onto the device without affecting the stored image. The additional images are produced by subjecting the device to radiation to which the variable impedance material is non-sensitive. In the preferred embodiment, the variable impedance materials are sensitive to only a portion of the wavelengths in the electromagnetic spectrum and at least one portion of the electromagnetic spectrum to which the material is non-sensitive lies within the visible spectrum. By the term electromagnetic spectrum it is intended to include the infrared rays, the visible spectrum, the ultraviolet rays and Xrays.

The nature of the invention will be more easily understood when it is considered in conjunction with the accompanying drawing wherein:

FIG. 1 is an enlarged fragmentary sectional view of an exemplary electroluminescent device of the present invention; and

FIG. 2 is a sectionalized perspective view of a display panel constructed in accordance with this invention in combination with appropriate projectors to provide an imaging system for the simultaneous viewing of two or more images.

It should be understood that in the figures the thickness of the layers, electrodes, etc., as well as other dimensions, have been greatly exaggerated to show the details of construction.

Referring to FIG. 1 is the imaging device comprises a plurality of spaced electrodes 11 mounted on a supporting substrate 12. Contacting each of the electrodes 11 is an electroluminescent material 13; specifically, the electroluminescent material 13 forms a layer overlying each of said electrodes 11. Electrical connections 15 are made to the electrodes 11 to enable the application of a voltage therebetween. Alternating electrodes are connected to one side of an alternating current voltage source 16 and the immediate electrodes are connected to the other side of the source. A layer of variable impedance material 14 is disposed over electroluminescent material 13. Under certain applications, it may be necessary to have a plurality of spaced electrodes (not shown) overlying the variable impedance material layer 14. Further, devices having contiguous electroluminescent and variable impedance layers sandwiched between tarnsparent electrodes can be used in the practice of the present invention.

The electroluminescent layer 13 can be formed of any known electroluminescent phosphor such as manganese activated zinc sulfide, copper activated zinc sulfide, etc. The electroluminescent material is preferably mixed with a transparent dielectric binder material, such as an epoxy or polyvinyl chloride resin, and then applied, by any known means, over the spaced electrodes. In general, the electroluminescent material layer has a thickness on the order of about several mils to about 10 mils and coatings of this thickness are transparent, or at least translucent, when subjected to sufficient radiant energy. That is, if a radiant energy image is projected onto the backside of an electroluminescent layer of the thickness described, a substantial proportion of the radiant energy can be viewed on the opposite side from the projector.

As a matter of convenience, the electroluminescent material has been shown in the form of a continuous layer. However, the electroluminescent material which lies in the spaces between electrodes does not produce substantial electroluminesence during the operation of the device. Consequently, such portions may be replaced by insulating material if desired. Thus, it is necessary that only the electrodes be coated with the electroluminescent material.

If it is desired to view the storage device from the side opposite the variable impedance material side or if it is desired to project the additional image(s) through this side, then supporting substrate 12 and spaced electrodes 11 should be transparent. A suitable substrate-electrode combination is optically transparent glass over-coated with thin optically transparent electrodes of tin oxide. The transparent electrodes may be produced by applying tin oxide, produced by the reaction of vapors of stannic acid, water, and methanol, through a suitable mask. In addition to glass as a supporting substrate, it should be noted that clear transparent plastics, such as Mylar are also acceptable materials.

If it is desired to view the stored image from the variable impedance material side of the panel or if it is desired to project the additional image(s) through this side, then the variable impedance material should be transparent or translucent to (1) the emitted light from the phosphor and (2) the projection light used in producing the nonstored image. In actuality, because of the thinness of the variable impedance material layer the difference between transparency and translucency is so slight as to be immaterial. In this latter structure, the panel can be fabricated on an opaque insulating base using opaque, for example metallic, electrodes and the images viewed by reflection. Suitable translucent variable impedance materials include thin layers of zinc oxide, lead oxide, cadmium oxide, Rochelle salt, etc. In addition, any spaced electrodes which are disposed over the variable impedance material should also be transparent or translucent.

The variable impedance material can be deposited upon the electroluminescent layer by any known process. The variable impedance material can be a ferro-electric material, such as Rochelle salt (potassium sodium tartrate); a field effect semiconductor, such as zinc oxide, lead oxide, cadmium oxide, and cadmium sulfide; a photoconductor such as cadmium selenide, selenium, and selenium alloys with minor amounts of arsenic or tellurium; or any other material which can advantageously be utilized as a control layer in the production of an electroluminescent storage device. Various dyes, sensitizers and/ or activators can be utilized to increase the optical response of the variable impedance material. For example, various dyes and sensitizers can be added to the material to extend or increase the spectral response of a photoconductor though care must be taken to leave the material relatively insensitive to at least one portion of the visible spectrum.

The electrode strips employed are merely convenient means for accurately selecting the length and cross-sectional area of the current path. Thus, by decreasing spacing between adjoining electrodes and/ or by increasing the thickness of the variable impedance material coating, one can increase the current therethrough for a given set of conditions, Additionally, the electrode strips may have any configuration as long as the modification of the impedance of the variable impedance layer continues to control the current flow between the electrodes.

The method of simultaneously producing two images for viewing purposes Will not be described in reference to each of the three basic types of storage devices previously described. A separate discussion of the operation of each exemplary storage device is believed to be helpful for a complete understanding of the advantages of the present invention.

In operation of the storage panel wherein a field effect semiconductor material is utilized as the variable impedance material, an alternating current voltage is applied between the spaced electrodes which is sufiicient to induce electroluminescence when the field efiect semiconductor material is at its low impedance state. For example, the deposition of negative electrostatic charge on the charge retention surface of a display panel having an ntype field effect semiconductor can be used to control the flow of current from electrode to electrode. Deposition of electrostatic charge increases the impedance of the field eifect semiconductor thereby reducing or interrupting the flow of current in adjacent areas. Reduction of current flow will cause a corresponding reduction in light output and if the current is lowered below that which is sufiicient to induce electroluminescence, luminescence will not occur and that particular portion of the storage device will appear dark. Conversely, the impedance is lowered and current flow increased as the charges are neutralized or removed from the charge retaining surface. Accordingly, by selectively placing a charge pattern on the charge retaining surface of the display panel an image may be produced and stored upon the device.

When it is desired to produce a white picture on a black background, an electrostatic charge is uniformly deposited over the entire charge retention surface. Neutralizing or removing a portion of the charge will cause current flow in adjacent areas thereby resulting in luminescence of the phosphor layer beneath the areas where charge has been neutralized or removed. A white picture on a black background can also be obtained by depositing a selected electrostatic charge pattern wherein dark background areas correspond to areas of charge deposition. Luminescence of the phosphor layer beneath those areas of the semiconductor layer where no charge resides will produce a white picture on a black background.

When it is desired to have a black picture on a White background, a selected electrostatic charge pattern is placed on the charge retention surface. This results in an increase in the impedance of the semiconductor thereby interrupting the flow of current in adjacent areas. When current flow falls below the level which is sufficient to induce electroluminescence, that portion of the storage device where the charge resides will appear dark, and a black on white picture will be obtained. Alternatively, a uniform electrostatic charge can be applied to the charge retaining surface and then a portion of the charge corresponding to the light background areas can be removed or neutralized to produce the desired result of a black picture on a white background.

A uniform electrostatic charge can be deposited on the surface of the field effect semiconductor and then a portion thereof removed to give the selected charge pattern or, in the alternative, the selected charge pattern can be deposited initially. For a complete description of the manner and means for creating a charge pattern on the surface of the field effect semiconductor, reference is made to Ser, No. 582,856 filed Sept. 29, 1966, which is incorporated herein by reference.

With the first image now stored on the unit, one or more additional images can be optically projected onto (or through) the unit by any suitable means. If the field effect semiconductor is not sensitive to the impinging radiation, regardless of whether it is a photoconductive insulator or not, the image stored upon the unit will not be affected and two or more images can be viewed simultaneously. If the field effect semiconductor also has photoconductive insulating properties, such as zinc oxide, then the input radiation for the additional images should not substantially affect the electrostatic charge which is stored on the surface thereof. That is, the input radiation should be in that portion of the visible spectrum to which the photoconductive insulator is substantially nonsensitive. Unsensitized zinc oxide, for example, is relatively insensitive to radiation above 4500 A. thus light below that wavelength can be used to store the first image upon the panel. Or zinc oxide can be dye sensitized with materials such as Rose Bengal (Eosin Y, Erythrosin and Fluorescein to increase the spectral sensitivity in the range of 4500-6000 A. The non-stored image can then be projected onto (or through) the panel with red light to which the dye sensitized zinc oxide will be substantially non-responsive,

As used in this application, the term field eifect semiconductor refers to a material which has the conductance thereof modified by applying an electric field perpendicular to the current flow thereby creating a region which effectively reduces the conducting cross-section of the semiconducting material. In the preferred embodiment, the field effect semiconductor material should be capable of retaining for substantial periods of time an electrostatic charge pattern on its surface and conducting current through the body thereof without substantially altering the surface charge pattern. When a single material has both of these physical properties it will be referred to as a storing field effect semiconductor. That is, the storing field effect semiconductor is capable of retaining an electrostatic charge pattern on its surface which then acts as the perpendicular electric field for modifying the conductance of the semiconductor material. Suitable materials exhibiting this combination of characteristics include zinc oxide, lead oxide, and cadmium oxide.

Additionally, many semiconductors which exhibit the field effect phenomenon can be adapted to the practice of this invention even though they are, initially, incapable of retaining an electrostatic charge pattern on their surface for the desired period of time. This modification is made by depositing a layer of insulating material on the side of the field effect semiconductor material opposite the side in contact with the electro-luminescent phosphor, the deposited electrostatic charge pattern residing, in this embodiment, on the insulator surface rather than on the surface of the semiconductor material itself. Typical semiconductors exhibiting the field effect phenomenon which can be modified by deposition of an insulator layer include cadmium sulfide, zinc sulfide, activated zinc sulfide, zinc oxide, cadmium selenide, etc. In the alternative, a barrier layer can be produced along the outer surface of the semiconductor material by suit ably doping the semiconductor to provide a p-n junction. The junction will act as a blocking layer preventing the passage of surface charge into the underlying material.

For brevity, the storing field effect semiconductor material will be referred to herein as the semiconductor material or the field effect semiconductor material, it being understood that the storage panel has an exterior surface which is capable of retaining an electrostatic charge pattern thereon for substantial periods of time.

-In the case of zinc oxide, besides substantially pure zinc oxide, a wide variety of zinc oxide compositions can be utilized which comprise zinc oxide dispersed in a nonconductive resin binder. The ratio of zinc oxide to binder can be in the range of 3/1 to 50/1. As previously noted, various dyes and sensitizers can be added to the composition to extend or increase the spectral response though care must be taken to leave the composition relatively insensitive to at least one portion of the visible spectrum.

If the electroluminescent device is of the type disclosed by Kazan et al. in US. 2,768,310 (i.e. the variable impedance material is a photoconductor) then the stored image is produced in the following manner. An alternating curent voltage is applied between the electrodes which is insufficient to cause enough current flow to produce electroluminescene of the phosphor layer. The stored image is then produced by increasing the conductivity of the photoconductive material by exposing that layer in image configuration to incident radiation. The reduction of impedance of the photoconductive material permits additional current flow which is sufficient to cause luminescene of the adjoining portions of the electroluminescent material. With this first image stored upon the imaging device, additional images can be produced thereon by optically projecting one or more images onto (or through) the panel. To avoid destruction or modification of the stored image, the incident radiation for the additional images should be of wavelengths to which the photoconductor is substantially non-sensitive.

When using the electroluminescent device of the type disclosed by Kazan in US. 2,905,849, the stored image is produced by utilizing a target comprising a transparent supporting substrate, a plurality of spaced electrodes on one surface of the supporting substrate, a layer of electroluminescent material overlying the plurality of electrodes, a layer of variable in the form of a ferroelectric layer impedance material overlying the layer of electroluminescent material, and a plurality of spaced conducting elements on the outer surface of the variable impedance material. Alternating electrodes which are on the surface of the supporting substrate are connected to one side of an alternating current voltage source with the intermediate electrodes being connected to the other side of the source. An alternating current is passed through the unit which is sufficient to induce the luminescence of the phosphor material. By superimposing a direct current voltage upon the alternating current voltage the reactance of the ferro-electric layer can be increased. A sufficient increase in the reactance of the ferro-electric layer is accompanied by a corresponding decrease in the potential drop across the electroluminescent layer resulting in the reduction and/or termination of emitted light from the electroluminescent layers. The direct current voltage is superimposed upon the alternating current voltage by modifying the potential of the conducting elements which are disposed upon the surface of the ferro-electric material. Any known means can be utilized to modify the potential of the conducting elements; this includes the electron gun means disclosed in the aforementioned patent as well as other means, such as corona discharge.

With the first image now stored on the unit, one or more additional images can be optically projected onto (or through) the unit by any suitable means. As previously noted, the input radiation should be of such quality as to not substantially affect the stored image. If it is desired to project the second image through the unit from the variable impedance material side or if it is desired to view the images from the variable impedance material side, then the variable impedance material, as well as the conducting strips disposed thereon, should be transparent or translucent to (1) the emitted light from the phosphor and (2) the projection light used in producing the non-stored image. A suitable transparent ferro-electric material is Rochelle salt (potassium sodium tartrate). Tin oxide is a suitable transparent material for the conducting elements.

The following example is given to enable those skilled in the art to more clearly understand and practice the invention. It should not be considered as a limitation upon the scope of the invention but merely as being illustrative thereof.

A glass plate about 12" long and about 12" wide and thick has a grid of transparent NESA glass conducting electrode strips formed thereon. Each NESA electrode strip extends the Width of the plate and has a width of about mils with a uniform spacing of about 10 mils. Coated over the electrode strips on the glass plate is a layer of about 2 mils thickness of zinc sulfide phosphor in an epoxy resin binder. Alternate connecting electrodes are connected to one side of an alternating current voltage source with the intermediate conducting electrodes connected to the other side of the voltage source. A 1 mil thick layer of undyed zinc oxide field effect semiconductor is coated over the phosphor layer. An alternating current voltage which is sufficient to induce electroluminescence of the phosphor layer when the field effect semiconductor material is at its low impedance state is applied between the spaced electrodes. A uniform electrostatic charge is disposed over the exposed zinc oxide surface causing complete termination of the emitted light. An input image is stored upon the device by subjecting the zinc oxide layer to radiant energy excitation in the wavelength range of 3,500-4,000 A. The input image is produced by using a conventional tungsten lamp slide projector having an ultraviolet transmitting filter (Corning type 754 glass filter). The input image remains stored upon the device for a substantial period of time. An interference filter (Optics Technology Company) transmitting only light of wavelengths greater than 5,000 A. is placed in front of the tungsten lamp projector resulting in the production and superimposition of a yellow non-stored image over the stored image on the electroluminescent device.

The undyed zinc oxide field effect semiconductor is not substantially affected by light having wavelengths greater than about 4,500 A.; accordingly, the input light for the second image does not affect the storage action of the zinc oxide layer as the radiation is not absorbed. However, because of the translucent nature of the device, this radiation can be seen on the supporting substrate side of the panel. In this procedure the panel acts as a type of viewing screen for rear-projecting images.

Thus, in the operation of the storage device, an image may first be stored on a panel, for example, a map or other display involving fixed information. While this image is being viewed, one or more additional images may be projected onto the panel and viewed simultaneously with the first stored image. However, upon terminating the radiation for the non-stored images, they will immediately disappear and will not be retained not stored by the panel.

When using a photoconductor-type electroluminescent device, it may be necessary to provide at least two optical projectors. One projector is utilized to produce the stored image while additional projectors are utilized to superimpose the non-stored images over the stored images. However, where the photoconductor will maintain a low conductivity level after the input signal is terminated, the stored image may persist for a suflicient period of time to allow using the same projector to produce the nonstored image by illumination with non-sensitizing radiation.

When dealing with the ferro-electric type-electroluminescent device, which requires electrical input to provide a stored image, it is only necessary to have a single light source which will project input radiation for a non-stored image onto the panel. Additional projectors to provide additional images can be used if desired.

When using the field effect semiconductor-type electroluminescent device, a single projector can be used to provide both the stored and non-stored images. sensitizing light emitted from the light source can be used to store an image on the panel in the manner previously described, but termination of the input radiation will not terminate the output image. Subsequently, a filter can be placed in front of the light source to provide non-sensitizing light which can then be projected onto the panel to give an additional non-stored image for simultaneous viewing. Multiple projectors can be used if desired. Further, reference is made to Ser. No. 582,856 for a more thorough discussion of other means for producing a stored image on this particular type of display panel.

Referring to FIG. 2, the imaging system 20* comprises a display panel 21 having a plurality of transparent spaced electrodes 22 disposed over one surface of glass faceplate 23 Overlying each of electrodes 22 is a layer of electroluminescent material 24 which in turn is covered with a layer of variable impedance material 25. Appropriate electrical connections 26 are made, as previously described, to connect electrodes 22 to an alternating current voltage source 27. Optical projectors 28 and 29 are positioned to illuminate the rear side of display panel 21. In operation, for example, when using a photoconductor as the variable impedance material, a first image of electromagnetic radiation of wavelengths to which the photoconductive material is sensitive can be projected by optical projector 28 onto the rear side of display panel 21. As long as the input light from the optical projector illuminates the display panel an intensified output image will be seen on the op posite side of the device. During this storage time, a second' image can be projected through the transparent unit from optical projector 29. If the electromagnetic radiation forming the illumination for this second image comprises wavelengths to which the photoconductor is non-responsive then the illumination will not affect the stored image and both images can be viewed simultaneously. New or additional stored or non-stored images can be projected onto the device as desired. When using a field effect semiconductor as the variable impedance material, then appropriate means (not shown) are required to produce a selected electrostatic charge pattern on the exposed surface of the panel. When using zinc oxide as the field effect semiconductor, the selected electrostatic charge can be produced by providing a uniform electrostatic charge over the entire applicable surface and dissipating a portion of that charge in response to sensitizing light emitted by optical projector 28. Projection of non-sensitizing light from projector 29 through the unit will produce a nonstored image which will not affect the stored image with the result that both images can be viewed simultaneously.

In the practice of this invention, it will also be possible to project a non-stored image onto the display panel, continuously view this projected image and modify such image until such time as the image desired is shown. The image can then be stored by switching to sensitizing radiation or by depositing an appropriate electrostatic charge pattern, as previously described. Alternatively, one can store an image and then continuously modify that image until such time as the desired image is shown on the panel.

While the invention has been described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the true spirit and scope of the invention.

It should be understood that the present invention is not dependent upon the exact nature of the electroluminescent materials or the variable impedance materials employed, rather any suitable material or structure can be employed, provided the advantageous results of this invention are not adversely affected.

All substitutions, additions, and modifications to which the present invention is readily susceptible, without departing from the spiirt and scope of this disclosure, are considered part of the present invention.

What is claimed is:

1. A method for the visual comparison of information comprising providing a display panel including a plurality of transparent spaced electrodes on one surface of a transparent supporting substrate, said electrodes being the only electrodes associated with said display panel, electroluminescent material overlying said spaced electrodes, and a layer of photoconductive material overlying said electroluminescent material; applying an alternating current voltage between said spaced electrodes which is insufficient to induce luminescence of said electroluminescent material; exposing said display panel to a first image of electromagnetic radiation of a wavelength to which said photoconductive material responds, said electromagnetic radiation being of sufiicient intensity to increase the conductivity of said photoconductive layer to at least the point where sufficient current will flow to induce luminescence of varying intensity in the exposed areas thereby controlling current flow between adjacent ones of said spaced electrodes and producing a stored image; and exposing said display panel to at least one additional image of electromagnetic radiation of a wavelength to which said photoconductive material does not respond so said stored image and said additional image can be reviewed simultaneously.

2. A method for the visual comparison of information comprising providing a display panel including a plurality of spaced transparent electrodes on one surface of a transparent supporting substrate, said electrodes being the only electrodes associated with said display panel, electroluminescent material overlying said spaced electrodes and a layer of variable impedance material selected from the group consisting of field-effect semiconductor material and photoconductor material overlying said electroluminescent material; producing and storing a viewable first image on said display panel; and projecting at least one additional image onto said display panel without affecting those factors which cause said first image to be stored so that first image and said additional image can be viewed simultaneously.

3. A method for the visual comparison of information comprising providing a display panel including a plurality of spaced transparent electrodes on one surface of a transparent supporting substrate, said electrodes being the only electrodes assoicated with said display panel, electroluminescent material overlying said spaced electrodes, and a layer of field-effect semi-conductor material overlying said electroluminescent material, said panel having an exposed charge-retaining surface adjacent at least one portion of said field-effect semiconductor material; forming an electrostatic charge pattern on at least a portion of said charge-retaining surface, said charge pattern being adapted to regulate the flow of current between adjacent ones of said spaced electrodes; passing current between said electrodes through said field-effect semiconductor material and said electroluminescent material to cause portions of said electroluminescent material to luminesce so that a first image of light and shadow is produced and maintained on said display panel; and projecting at least one image of electro-rnagnetic radiation on to said display panel without affecting those factors which cause the storage of said first image so that said first image and said projected image can be viewed simultaneously.

4. The method of claim 3 wherein the field effect semiconductor material has photoconducting properties and the electromagnetic radiation of said projected image is of a wavelength to which said field effect semiconductor does not respond.

5. The method of claim 4 wherein the field effect semiconductor material is zinc oxide and the electromagnetic radiation utilized for the formation of said projected image comprises wavelengths above 4500 A.

'6. The method of claim 3 wherein said field-effect semiconductor material is a storing field-effect semiconductor material, said charge-retaining surface corresponding to the exposed surface of said storing field-effect semiconductor material substantially parallel to the interface between said electroluminescent material and said fieldefiect semiconductor material.

7. The method of claim 3 further including an insulating layer over-lying said field-effect semiconductor material, said insulating layer having an exposed surface corresponding to said charge-retaining surface substantially parallel to the interface between said insulating layer and said field-effect semiconductor layer.

8. The method of claim 7 where said insulator is a photoconductive insulator.

9. An imaging system comprising a display panel including a plurality of spaced transparent electrodes on one surface of a transparent supporting substrate, said electrodes being the only electrodes associated with said display panel, electroluminescent material overlying said spaced electrodes, and a layer of variable impedance material selected from the group consisting of field-effect semiconductor material and photoconductive material overlying said electroluminescent material; means for producing a stored first image on said display panel; and means for superimposing at least one non-stored image on said display panel simultaneous with the storage of said first image without affecting those factors which cause said first image to be stored so said first image and said non-stored image can be viewed simultaneously.

10. The imaging system of claim 9 wherein the variable impedance material is a photoconductor.

11. The imaging system of claim 10 wherein said means for producing a stored first image comprises means to expose said photoconductor to an actinic electromagnetic radiation pattern, whereby current flow between adjacent ones of said spaced electrodes is controlled to produce said first image.

12. The imaging system of claim 9 wherein the variable impedance material is a field effect semiconductor material.

13. The imaging system of claim 12 wherein the field effect semiconductor material is zinc oxide.

14. The imaging system of claim 12 wherein said fieldelfect semiconductor is a storing field-effect semiconductor material, said field-effect semiconductor having an exposed charge-retaining surface adapted for the formation of an electrostatic charge pattern thereon, said charge pattern serving to control the flow of current between adjacent ones of said spaced electrodes.

15. The imaging system of claim 12 further including a photoconductive insulating layer overlying said field-efiect semiconductor material, said photoconductive insulating layer having an exposed surface adapted for the formation and retention of an electrostatic charge pattern thereon, said charge pattern serving to control the flow of current between adjacent ones of said spaced electrodes through said electroluminescent material and said fieldeifect semiconductor material.

16. The imaging system of claim 12 wherein said means for producing a stored first image compises means to form an electrostatic charge pattern serving to control the flow of current between adjacent ones of said spaced electrodes whereby said first image is produced and stored.

References Cited UNITED STATES PATENTS 5/1957 Kazan 250213 X 9/1959 Kazan 250-213 6/1961 Lieb 250213 X 3/1965 McNaney 250--213 X 4/1966 Kazan 250213 9/1967 Martel 250-213 US. Cl. X.-R. 

